1. Field of the Invention
The present invention refers to a crossbar for heald-carrying frames comprising an improved attachment of the heald-carrying plate to said crossbar.
2. Description of the Related Art
As is well known to skilled people in the field, the heald-carrying frame is a device used in weaving looms to achieve the shifting of groups of warp yarns, thanks to the alternate movement thereof in a vertical plane perpendicular to the weaving plane. In the weaving loom a certain number of heald-carrying frames is arranged, the greater the degree of complexity of the pattern to be woven on the fabric, the higher the number of such frames, and the individual frames are controlled by a weaving machine in order to achieve a preset pattern on the fabric.
Each heald-carrying frame comprises a rectangular rim consisting of two side elements making up the guides for the alternate sliding of the frame, and of two horizontal elements, called crossbars, on whose opposite inner sides a plurality of thin steel rods is fastened, provided with an intermediate eye for one or more warp yarns to pass through. Such rods are called indeed healds.
The two side elements and the two crossbars must further be mutually fastened at a right angle, in the angular positions of the frame, so as to provide a rigid and stable structure, capable of withstanding the high stress levels which the frame undergoes during its rapid, and sometimes very rapid, alternate movement within the loom.
Over the last few years continuous efforts have been made to improve the performance of the above-said devices, in particular towards reducing the mass and increasing the useful life thereof. Such objects are of course in conflict, since a lighter structure is more prone to fatigue breaking, which typically represent the most frequent cause of breaking of the devices undergoing continuous and rapid inversions of inner stresses, as indeed in the case of heald-carrying frames. In order to reduce the incidence of this problem, a number of attempts have been made to form the crossbars using, instead of the conventional aluminium-based light alloy metal sheets or light alloy metal sheets made of other low specific-weight metals, composite materials made of different types of fibres, synthetic resins and foam materials, all materials which are less affected by the problem of fatigue breaking over metallic materials. However, the much higher costs of this type of heald-carrying frames has not allowed a sufficiently wide diffusion thereof yet and the frames in metallic materials consequently still represent a considerable portion of the market.
Such frames, however, have—as shown—an excessively short useful life, especially in connection with the inherent fragility induced in a heald-carrying frame by the system fastening the heald-carrying plates to their respective crossbars. As a matter of fact, the majority of the frames on the market currently provides a mutual fastening by means of rivets of the above-said components. This system—which is certainly very inexpensive, safe and allows quick assembly, and which is consequently currently preferred—however, has remarkable and noticeable disadvantages, particularly in terms of its dramatic reduction of the fatigue-withstanding properties of the crossbars.
As a matter of fact, as is known, the operations of crossbar drilling, and of the subsequent upsetting of the rivet on the respective holes for the fastening of the heald-carrying plate, induce very strong localised stresses in the metallic profile making up the crossbar. These, understandably, drastically reduce the fatigue breaking limit of the crossbar, and as a result cause a very short useful life of the heald-carrying frames.
This problem then becomes the more serious the faster the looms whereon the heald-carrying frames are mounted; as a matter of fact, the higher speed implies greater dynamic stresses and a higher number of cycles of alternate stresses per time unit, both conditions reducing the fatigue limit. In the more recent air-jet looms, wherein weaving speeds are extremely high, the problem of breaking frequency or of scheduled replacement of the heald-carrying frames has hence become such as to negatively affect the entire weaving operation.